1. Field of the Invention
The present invention relates to a semiconductor memory device, such as an MRAM (Magnetic Random Access Memory), which uses magnetic materials including tunneling magnetoresistive elements (hereinafter called “TMR elements”) or giant magnetoresistive elements (hereinafter called “GMR elements”).
2. Description of the Related Art
Conventionally, a TMR element 900 is constructed as shown in FIGS. 1A and 1B.
In FIGS. 1A and 1B, the TMR element 900 has a pin layer 902, a tunneling insulator layer 903 and a free layer 904 laminated in order on a diode 901 and is connected in series to the diode 901.
The pin layer 902 is formed of a magnetic material and the direction of its magnetization is fixed at the time it is formed.
The free layer 904 is likewise formed of a magnetic material (e.g., NiFe) but in such a way that its magnetization is reversed by the current that is generated by a bit line 905 connected to the diode 901 and a word line 906 connected to the free layer 904 and passes the TMR element 900 in the up and down direction. “1” (FIG. 1A) or “0” (FIG. 1B) is assigned depending on the direction of the magnetization of the free layer 904.
The magnetic material in use here for the free layer 904 is so selected as to have such a property and shape that the magnetization is easily reversed when a magnetic field is applied in an obliquely rearward direction.
A semiconductor memory device which uses TMR elements with such a structure as memory cells is constructed, for example, as shown in FIG. 2.
In FIG. 2, a semiconductor memory device 910 comprises a plurality of memory cells 911 laid out in a matrix form, a plurality of bit lines (BL) 912 extending in parallel vertically under the individual memory cells 911, a plurality of word lines (WL) 913 extending in parallel horizontally above the individual memory cells 911, an X-side write current source circuit 914, an X selector 914a, a Y-side write current source circuit 915, a Y selector 915a, a terminating power supply circuit 916 an X termination circuit 916a and a Y termination circuit 916b. 
Each memory cell 911 is constituted by the above-described TMR element 900 and a current is made to flow in the memory cell 911 by the associated bit line 912 and word line 913 so that the direction of the magnetization of the free layer 904 can be reversed.
According to the semiconductor memory device 910 with such a structure, as one memory cell 911 is selected and a current is made to flow between the bit line 912 and word line 913 associated with that memory cell 911, it is possible to allow the current to flow only in the selected memory cell 911 and reverse the direction of the magnetization to write data of “0” or “1”.
The principle of the data writing operation will be discussed by referring to FIGS. 3A through 3C.
The magnetization of the free layer 904 of the TMR element 900 is reversed when a magnetic field of an intensity greater than a certain level is applied to the free layer 904. The characteristic curve of the magnetic field is called an asteroid curve.
Magnetic fields (see FIGS. 3B and 3C) which fit inside the asteroid curve are formed in the memory cells 911 on the selected bit line 912 or the selected word line 913, and a current which makes the combined magnetic field outside the asteroid curve as shown in FIG. 3A is set in the selected memory cell 911.
The principle of a data reading operation will be discussed by referring to FIG. 4.
As each TMR element 900 is equivalent to a variable resistor whose resistance changes in accordance with whether the value of data is “0” or “1”, the semiconductor memory device 910 is expressed by an equivalent circuit shown in FIG. 4 because of the diode 901 being connected in series to the TMR element 900.
Because a voltage of 1.2 V is applied to an unselected bit line 912 and a selected word line 913, therefore, the current flows only in the selected memory cell 911. A current value sense amplifier 917 pulls in the current in such a way that a voltage of about 0.3 V is applied between the pin layer 902 and the free layer 904 of the TMR element 900 with respect to a threshold value of 0.7 V of the diode 901. If this current value, when measured, is greater than a reference current set beforehand, data is judged as “0”, and if the current value is smaller than the reference current, data is judged as “1”.
A semiconductor memory device which uses transistors in place of the diodes 901 is known as disclosed in, for example, U.S. Pat. No. 6,191,989, and a semiconductor memory device which uses neither diodes nor transistors is also known as disclosed in, for example, U.S. Pat. No. 6,188,615.
Although those semiconductor memory devices differ in operations in read mode, their operations in write mode are carried out in the same way as the writing operation of the semiconductor memory device 910 that uses the diodes.
A conventional MRAM cell is constructed as shown in FIGS. 5A and 5B.
In FIGS. 5A and 5B, an MRAM cell 950 is constructed in such a way that a tunneling insulator layer 951 is held by a plurality of ferromagnetic materials, i.e., a fixed ferromagnetic layer 952 and a free ferromagnetic layer 953.
The fixed ferromagnetic layer 952 is formed of a material which has a large coercive force and is designed in such a way that magnetization is fixed in one direction by magnetic coupling or the like of the material with an antiferromagnetic material.
The free ferromagnetic layer 953 is designed in such a way that the magnetization can be reversed by the action of an external magnetic field or the like.
This structure allows the MRAM cell 950 to be stable when the magnetizations of the fixed ferromagnetic layer 952 and the free ferromagnetic layer 953 are parallel or antiparallel to each other and to store information of “0” (FIG. 5A) and information of “1” (FIG. 5B) in the respective two cases.
In the state of “0” or the parallel state, the tunnel current is large, whereas in the state of “1” or the antiparallel state, the tunnel current is small. By detecting the difference between the values of the tunnel currents, therefore, information of “0” or “1” stored in the MRAM cell 950 can be read out.
In case where a semiconductor memory device is constructed to have a memory cell array comprising MRAM cells with such a structure, writing and reading to and from the MRAM cells as individual memory cells can be performed in manners similar to those of the above-described semiconductor memory device 910.
While data writing to each memory cell 911 in the semiconductor memory device 910 is carried out by the magnetic field that is formed by orthogonal currents flowing through the bit line 912 and word line 913 as shown in FIG. 6, data writing cannot be done if the write currents are too small. If the write currents are too large, data is not written in not only the selected memory cell 911 but also the adjoining memory cells 911 connected to the same bit line 912 and the same word line 913 in some cases.
It was therefore necessary to accurately set the values of the currents flowing through the bit line 912 and word line 913 at the time of writing data.
While the asteroid curve depends on the film thickness of the magnetic material, the film thickness has a distribution in the surface of a semiconductor wafer at the time of manufacture and thus varies memory cell by memory cell.
Further, the characteristics of the write current source circuits 914 and 915 would vary chip by chip and it was not possible to completely eliminate the variation.
The variations in the film thickness and the characteristics of the write current source circuits reduce the write margin of each memory cell 911 of the semiconductor memory device 910 and lower the yield of the memory cells 911.
Because the asteroid curve has a temperature dependency, the reversed magnetic field (minimum write current) generally becomes smaller as the temperature gets higher. FIG. 7 shows the results of measuring the reversed magnetic field of permalloy with a size of 1 μm×2 μm and a thickness of 5 nm at 25° C., 75° C. and 125° C. It is apparent from the diagram that as the temperature rises, the reversed current of the magnetic film becomes smaller at a rate of about 2%/10° C.
Generally, the operation guaranteeing temperature of a semiconductor device is about 75° C. or lower, but the write current at 75° C. in FIG. 7 is dropped about 10%. The use of the write current at room temperature (25° C.) directly at a high temperature therefore causes disturbance in unselected memory cells. At that time, the current driving performance of the write current source circuit falls with a rise in temperature, so that while the write current decreases slightly, not large enough to follow up a reduction in reversed current. Such a reduction in reversed current which is originated from a temperature rise becomes more notable as the miniaturization of memory cells goes further.
It is also known that a rise in temperature reduces the read margin. It is generally known that the resistance R and conductance G of a TMR element have voltage dependencies as shown in FIGS. 8A and 8B and have temperature dependencies as shown in FIGS. 8C and 8D.
Because the MR ratio and the current difference also have temperature dependencies as shown in FIGS. 8E and 8F, therefore, the read margin drops as the temperature rises.
The read current in, for example, an MRAM is a tunnel current between magnetic materials, so that as the temperature rises, the magnetization of the magnetic film is reduced and the tunneling probability is increased by thermal excitation. This increases the tunnel current and abruptly decreases the magnetoresistance ratio, thus reducing the read margin. Such a reduction in read margin would have a greater temperature dependency as the miniaturization of the memory cells would get finer.